Field of the Invention
The present invention relates to a magnetic-disk glass substrate and a magnetic disk.
Background Information
Nowadays, personal computers, digital versatile disc (DVD) recorders, and the like have a built-in hard disk drive (HDD) for data recording. In particular, a magnetic disk in which a magnetic recording layer is provided on a glass substrate is used in a hard disk drive that is used in a device premised on portability, such as a notebook-type personal computer, and magnetic recording information is recorded on or read from the magnetic recording layer with a magnetic head that flies slightly above the surface of the magnetic disk. A glass substrate is unlikely to be plastically deformed compared with a metal substrate (aluminum substrate) and the like, and thus is preferably used as a substrate of the magnetic disk.
Moreover, the density of magnetic recording has been increased to meet the demand for an increase in the storage capacity of hard disk drives. For example, the magnetic recording information area (recording bit) has been made smaller using a perpendicular magnetic recording system that causes the direction of magnetization in the magnetic recording layer to be perpendicular to the surface of the substrate. A magnetic disk of the perpendicular magnetic recording system is obtained by forming an adherent layer, a soft magnetic layer (soft under layer. SUL), a base layer, a magnetic recording layer, a protecting layer, a lubricant layer, and the like in this order on a metal substrate or a glass substrate, for example. Employing the perpendicular magnetic recording system makes it possible to increase the storage capacity per disk substrate. Also, in order to further increase the storage capacity, the distance between the recording and reproducing element portion and the magnetic recording layer is made extremely short by causing the element portion of the magnetic head to project farther, thus further improving the accuracy of the recording and reproducing of information (improve the S/N ratio). It should be noted that such control of the recording and reproducing element portion of the magnetic head is called a dynamic flying height (DFH) control mechanism and a magnetic head equipped with this control mechanism is called a DFH head. A magnetic-disk glass substrate that is used in an HDD in combination with such a DFH head is produced such that the surface unevenness of the substrate is extremely small, in order to prevent the substrate from colliding or coming into contact with the magnetic head and the recording and reproducing element portion that projects farther therefrom.
On the other hand, it is known that the surface shape of a magnetic-disk glass substrate affects the crystalline orientation dispersion (Δθ50; deviation of crystals from the perpendicular direction) of a specific crystal plane, such as a Co (002) face or an Ru (002) face, of magnetic particles in the magnetic recording layer formed on the substrate. Δθ50 is calculated as a half-value width of a peak in the case where θ/2θ is measured with an X-ray diffractometer, a 2θ value is measured from the peak top of the crystal plane of the magnetic recording layer and a θ scan is performed while fixing the 2θ. The crystalline orientation dispersion Δθ50 is an index indicating the dispersion of an axis of easy magnetization, and the smaller this value is, the better. By improving the Δθ50 (i.e., by bringing the Δθ50 close to zero), it is possible to obtain excellent magnetic properties and to improve the signal-to-noise ratio (SNR), and therefore, it is possible to further increase the recording density.
Regarding the crystalline orientation dispersion (Δθ50), JP 2009-140584A describes a magnetic-disk glass substrate that is prepared not based on a surface roughness Ra but so as to have a root mean square value of inclination angles that is smaller than or equal to a predetermined value (for example, 5 degrees or less, and more preferably 3 degrees or less) in order to improve the crystalline orientation dispersion (Δθ50) and the SNR of the magnetic recording layer.
Moreover, JP 2008-293552A describes a magnetic-disk glass substrate in which the surface roughness Ra satisfies the relationship Ra≤0.15 nm and the average inclination angle is set to be 2 degrees or less. Using this substrate makes it possible to reduce Δθ50 and to reduce the medium noise (evaluated at a linear recording density of 825 kbpi using a TMR head of 130 Gbpsi). It should be noted that in the case where the linear recording density is 825 kbpi, a recording bit length (hereinafter, indicates a calculated value based on linear recording density) is about 30 nm.